1. Field of the Invention
This invention relates to a process for industrially advantageously producing aluminum by electrolyzing aluminum chloride in molten salt.
2. Description of the Prior Art
Aluminum has been heretofore produced by a so-called Hall-Heroult process wherein aluminum oxide (alumina) is dissolved in an elecyrolytic bath of a metal fluoride molten salt consisting predominantly of molten cryolite and is electrolyzed in the electrolytic cell. However, in the Hall-Heroult process, in principle, such large amounts of electric energy are required to electrolytically reduce alumina that, in fact, the electric power required to produce one ton of aluminum is 15,000kwh or more; therefore, the development of a process for producing aluminum with reduced electric energy consumption is highly desirable.
As a prospective energy saving aluminum producing process to take the place of the Hall-Heroult process, there is known an aluminum chloride electrolyzing process wherein aluminum chloride is dissolved in a molten salt bath of alkali metal chloride such as NaCl or KCl (sometimes accompanied by small additions of alkaline earth metal chloride) and is electrolyzed. Although this aluminum chloride melting electrolyzing process has definite advantages in that it can be operated at an electrolyzing temperature of about 700.degree. C., which is about 300.degree. C. lower than in the Hall-Heroult process, and in that, as the anodic reaction releases chlorine, the graphite electrode used for the anode will not be consumed, it has been long ignored in the industry for the reasons that it is troublesome to handle aluminum chloride and chorine gas at high temperatures and that no adequate bath-resistant material was industrially available. However, an aluminum chloride electrolyzing process (the so-called ALCOA process) employing a new electrolyzing apparatus and an electrolytic bath having new composition has been recently suggested by ALCOA (Aluminum Company of America), U.S.A. (U.S. Pat. No. 3,822,195) and has quickly received industrial attention.
In this ALCOA process, the electrolysis is carried out at a bath temperature of about 700.degree. C., an electrode distance of about 15mm and a current density of about 1 ampere/cm.sup.2 in an electrolytic bath composed of a molten salt of an AlCl.sub.3 -- LiCl -- NaCl system in which the LiCl is mixed at a high concentration by means of an electrolytic cell made by horizontally arranging bi-polar electrodes formed of carbon (graphite) electrode plates with a proper clearance between them in a cell lined with a refractory material having a nitride base so that chlorine gas may be produced on the anode surface and molten metallic aluminum may be produced on the cathode surface. The special features of this process include the use of a special material high in the fireproofness and bath corrosion-resistance corrosion is used for the electrolytic cell container, the introduction of LiCl high in electric conductivity in the molten salt electrolytic bath composition to reduce the voltage drop of the bath, and a reduction in the distance between the electrodes to lower electric power consumption.